Bulk ceramic copper oxide superconductors can experience large stresses in many applications. A useful design criterion for the mechanical properties of this type of superconductor is the ability to withstand a strain of 0.2%. The high-temperature superconductor YBa.sub.2 Cu.sub.3 O.sub.x (123) is generally weak because of its low fracture toughness and the presence of relatively large flaws and thus, in general, does not have the desired flexibility. The fracture toughness (K.sub.IC) of 123 is generally only =1-2 Mpa(m)e.sup.0.5. Fracture toughness is related to fracture strength (.sigma..sub.f) by EQU K.sub.IC =Y .sigma..sub.f (.pi.c).sup.0.5,
where Y is a geometric factor and c is the size of the critical flaw. Therefore, if the flaw population remains constant, increases in K.sub.IC should impart proportional increases in strength. The flexibility of a brittle wire or tape is directly related to its strength by EQU .sigma..sub.f =(r/p)E,
where r is the radius of the wire, p is the bending radius, and E is the Young's modulus. Thus, even in the absence of ductility, significant increases in K.sub.IC may help allow for fabrication of robust, flexible wires and tapes. This invention is applicable generally to the ceramic copper oxide superconductors, including the 123 superconductor and the superconductors having various rare earth substitutions for yittrium. In addition, the invention pertains to other presently known ceramic copper oxide superconductors, namely Bi-Sr-Ca-Cu-O.sub.x (several different phases are known) and Tl-Ba-Ca-Cu-O.sub.x (several different phases are known). When we speak of copper oxide superconductors we mean these three systems.
K.sub.IC of ceramics can be enhanced by several means. Some mechanisms, such as promotion of a tortuous crack path, lead to only modest improvements. Others, such as microcrack toughening, can be effective, but would be of little use to 123 superconductor or the other ceramic copper oxide superconductors owing to a concomitant degradation of electrical properties. The mechanisms that show the most promise for toughening high-temperature superconductors involve addition of second-phase particles. It has been shown that addition of 15 to 20 volume percent Ag particles nearly doubles K.sub.IC and can improve strength. The improvements are due to the ductility of the Ag and to a favorable residual stress state in which the brittle ceramic is in compression and the ductile Ag is in tension.
It has been found that dispensing an agent throughout a ceramic copper oxide superconductor, if the agent is properly chosen and coated, improves the toughness and strength of the superconductor without degrading the superconducting properties. The dispersed agents are ZrO.sub.2, Al.sub.2 O.sub.3, MgO, Al.sub.2 O.sub.3.SiO.sub.2 (mullite) and SiO.sub.2. The preferred agents are ZrO.sub.2, Al.sub.2 O.sub.3 and MgO. The agents commercially are available in various physical forms, such as particles, whiskers, platelets and continuous fibers. When ZrO.sub.2 particles are used, the tetragonal ZrO.sub.2 must be employed, whereas ZrO.sub.2 whiskers and continuous fibers need not be tetragonal. Al.sub.2 O.sub.3 is available in whiskers, fibers and platelets and any such form or forms is acceptable. MgO is available in whiskers and continuous fibers, either of which is satisfactory as well as mixtures of both forms. Al.sub.2 O.sub.3.SiO.sub.2 is available in continuous fibers, whiskers and platelets, while SiO.sub.2 is available in continuous fibers and whiskers.
Generally, it has been found that the agent should be present in a concentration of at least about 10 mole percent of the superconductor and not more than about 30 mole percent. The preferred concentration is about 20 mole percent. Various compounds may be used to protect the agent and prevent deleterious chemical reaction with the superconductor. The coating compounds are selected from the phase diagram for each of the superconductor systems. The selected compounds have the properties of being entirely chemically compatible with the respective superconductor. No chemical potential exists between the superconductor and the compound and there is no thermodynamic driving force for interdiffusion. In addition, the compounds listed have been found to be less reactive with foreign oxide additives (e.g., ZrO.sub.2 or MgO) than would be the superconductor itself. The coating compounds are Y.sub.2 BaCuO.sub.5 (otherwise known as 211), Ca.sub.2 CuO.sub.3, Sr.sub.2 CuO.sub.3, (Ca,Sr).sub.1 CuO.sub.2 and CaCuO.sub.2. Specifically, the 211 compound is used to protect agents dispersed in the YBa.sub.2 Cu.sub.3 O.sub.x superconductor. Ca.sub.2 CuO.sub.3 or Sr.sub.2 CuO.sub.3 or (Ca, Sr).sub.1 CuO.sub.2 or mixtures thereof are used to protect agents dispersed in Bi-Sr-Ca-Cu-O.sub.x ceramic copper oxide superconductors, while Ca.sub.2 CuO.sub.3 or CaCuO.sub.2 or (Ca, Sr).sub.1 CuO.sub.2 mixtures thereof are used to protect agents dispersed in Tl-Ba-Ca-Cu-O.sub.x ceramic oxide superconductors.